To date, a variety of optical projection systems have been proposed. Each of these display systems typically includes (1) an input polarizer, (2) one or more spatial light modulators, and (3) one or more output analyzers. An input polarizer linearly polarizes unpolarized light. One type of input polarizer is a polarizing beam splitter ("PBS"), which polarizes unpolarized light by splitting it into transmitted P-polarized light and reflected S-polarized light. P-polarized light is light that is parallel to the plane of incidence (which is defined by the incident and reflected rays), while S-polarized light is light that is perpendicular to the plane of incidence.
A spatial light modulator (SLM) receives the light that an input polarizer linearly polarizes. An SLM often includes an array of picture elements (also called pixels) that the SLM individually controls to modulate the light passing through the pixels. An SLM is typically formed by positioning a layer of liquid crystal material between two electrodes. One of the electrodes is segmented into an array of pixel electrodes to define the pixels of the SLM, while the other electrode is usually not segmented.
There are two varieties of SLM's: reflective and transmissive. In both varieties, the direction of an electric field applied between each pixel electrode and the other electrode determines whether the corresponding pixel changes the polarization of light falling on the pixel. Hence, in both varieties, the incident light is modulated by changing the polarization of light falling on certain pixels while leaving unchanged the polarization of the light falling on other pixels.
An output analyzer receives the light transmitted or reflected by an SLM. Output analyzers are polarization-selective devices similar to the input polarizers. Polarizing filters and PBS's are two types of output analyzers. An output analyzer allows a certain polarization state of the light to pass, while discarding the remaining polarization states. Hence, output analyzers are placed at the outputs of SLM's to obtain the pattern of modulation of the SLM's, and thereby generate images. An observer will not perceive an image unless an analyzer follows an SLM, because the SLM does not attenuate the incident beam of light, but rather simply modulates the lights polarization state.
Projection displays generate color images by modulating, analyzing, and combining component color bands. Display devices typically use a few component colors (such as the primary additive colors, red, green or blue) to generate a multitude of colors for display. A component color band is a portion of the light spectrum corresponding to a component color. When all the component color bands are added, they produce white light. Conversely, component color bands can be extracted from white light.
To generate color images, projection displays not only use input polarizers, SLM's, and output analyzers, but they also use other devices. For instance, color projection systems often either use (1) a light source for each component color band (e.g., three light sources for the three primary additive colors, red, green, and blue), or (2) a single source of white light with a prism or other color separation device that separates incident white light into component color bands (e.g., into red, green, and blue light).
The component color bands are then used to illuminate one or more SLM's, which modulate the incident light for each color band. The modulated color bands are then recombined to produce a full-color image. The recombination may take place sequentially or simultaneously.
I. Color-Field Sequential Display Systems.
Color-field sequential systems create an image by sequentially projecting red ("R"), green ("G"), and blue ("B") images. FIG. 1 presents one prior art color-field sequential system. This display system 100 uses a mechanical color filter wheel 105 positioned between a light source 110 and a light valve 115 (which includes an SLM and an analyzer).
As shown in FIG. 2, the filter wheel 105 is divided into three filter sections, each acting as a pass filter for one of the three primary additive colors. By rotating the filter wheel, successive red, green, and blue light are generated to illuminate the light valve. The light valve is then modulated to generate successive red, green, and blue images. The eye-brain system fuses the successively-projected color images into a single blended polychromatic image, if the eye is stationary and the successive color patterns are projected at a high rate.
The eye, however, is not always stationary and often moves, and this movement can cause the viewer to see artifacts, called color sequential artifacts ("CSA"). For instance, the viewer might see spurious images (such as flashes of red, green, or blue light). CSAs are not only annoying, but also present safety concerns (e.g., they may cause epileptic attacks).
Increasing the projection rate of the images can minimize color sequential artifacts. However, at high rates, the mechanical color filter 105 does not operate reliably and introduces noise and vibration into the system. Electronic color switches can be used in place of the mechanical filter 105, but the electronic switches require complicated processing and driving circuitry, and are somewhat inefficient at their high switching rates. Finally, sequential system 100 does not generate good color contrast because its light valve 115 cannot be cost-effectively designed to operate perfectly for each of the three generated color bands.
II. Simultaneous Projection Display Systems.
Simultaneous projection display systems create a color image by optically superimposing multiple partial-color images to the same location. In addition to using light sources, input polarizers, color-separating devices, SLM's, and output analyzers, simultaneous projection systems also use color-recombining devices (such as dichroic prisms) to recombine each of the component color images in a coordinated way.
Simultaneous projection systems may be divided into single-pass and double-pass systems. Double-pass systems use the same device for both the color separation and recombination operations, while single-pass systems use different devices for these operations.
A. Single-Pass Systems.
FIG. 3 presents one prior art single pass system. The light from the light source is separated into three color bands using dichroic filters. A separate light valve (formed by an SLM and an output analyzer) modulates each color band. The modulated color bands are then recombined using dichroic filters.
There are several disadvantages to this architecture. For example, this system is somewhat bulky and relatively expensive since it uses many components. Also, its projection lens is complex and costly since it needs a projection lens with large back-focal length due to the relatively large distance from the panel to the lens. The dichroic filters used for the recombination operations also introduce aberrations and distortion in the generated images.
FIG. 4 presents another prior art single-pass system. This system receives R, G, and B light either from three sources of light (as shown in FIG. 4), or from a color-separator (not shown) that separates these different color bands from white light. System 400 also utilizes three PBS's 420, 425, and 430. These PBS's serve as input and output polarizers. Specifically, the PBS's initially receive unpolarized light from light sources 405, 410, and 415. They transmit the P-polarized light out of the system, while reflecting the S-polarized light towards the SLM's 435, 440, and 445.
The SLM's then modulate and reflect the received light back to the PBS. On the second pass through, the PBS's serve as output polarizers (i.e., output analyzers). The analyzers (1) reflect and thereby reject the S-polarized light (corresponding to the light having a polarization that the SLM's did not change), and (2) transmit the P-polarized light (corresponding to the light having a polarization that the SLM's changed). The dichroic prism 450 receives the color images output from the analyzers and combines these images into a single polychromatic image. Projection lens 460 then projects this image on a screen.
The design and construction constraints on this system are considerably relaxed because each pair of PBS's and SLM's is tightly coupled and operates over a narrow color spectrum. Also, the recombination prism does not convert the polarization of the light because the light passing through it only has a single polarization orientation--in this case, P-polarized.
This system, however, uses many components. For instance, it either needs three different light sources, or it needs a color-separating device different than the recombination prism. As a result, this system is somewhat bulky and relatively expensive.
B. Double-Pass Systems.
Unlike single-pass systems, double-pass systems use one device (e.g., one dichroic prism) for both the color separation and recombination operations. Hence, double-pass systems are typically smaller and less expensive.
FIG. 5 presents one prior art double-pass projection display system. This system 500 includes a light source 505, a PBS 510, a prism 515, and three SLM's 520, 525, and 530. The light source 505 supplies unpolarized light to the PBS 510. This PBS serves as both the input polarizer and the output analyzer. As the input polarizer, the PBS polarizes the unpolarized white light that it receives from the light source 505 by transmitting P-polarized light out of the system (and thereby discarding this polarization), while reflecting S-polarized light towards the prism.
The dichroic prism 515 then separates the S-polarized white light into its color components, and directs each color light to the corresponding color SLM. The SLM's then modulate and reflect the received light. The reflected light includes both S-polarized light (corresponding to light having a polarization that the SLM's did not change) and P-polarized light (corresponding to light having a polarization that the SLM's changed).
The light reflected by the SLM's then enters the prism, which now combines the modulated color light and supplies the combined light to the PBS. On the second pass through, the PBS serves as the output analyzer that (1) reflects and thereby rejects the S-polarized light, and (2) transmits the P-polarized light. The projection lens then receives the P-polarized light from the analyzer and focuses this light on the screen.
The design and construction constraints on this system are considerable because the PBS operates as the analyzer for all three-color bands, and therefore must meet exacting performance requirements over the entire color spectrum. It is quite difficult to have the PBS perform optimally over the entire color spectrum. The PBS typically is optimized for one or two of the additive colors, which causes the PBS to offer poor contrast and poor dark states for the third additive color.
A high degree of scattered light also exists in the dichroic prism because all the light reflected by the SLM's is directed through the prism. This, in turn, increases the performance requirements on the prism. In addition, the light passing through the prism has both S and P polarization. This causes the prism to introduce polarization conversion. Specifically, when both S and P light traverse through the prism, the prism rotates the polarization of the S and P light and/or introduces ellipticity into the polarization state.
Polarization conversion then contaminates the analyzing operation performed by the PBS on the second pass. For instance, if the polarization conversion causes S-polarized light from a pixel (an "OFF" or dark pixel) to slightly rotate so that it now has a P-polarized component, then the PBS on the second pass does not reject all the light for that pixel and allows the P-polarization component to pass. Hence, the polarization conversion causes light to leak into the dark pixels and reduces the contrast and brightness of the bright pixels.
Therefore, there is a need in the art for a double-pass system that generates good dark states and good color contrast. There is also a need for a double-pass system that avoids leakage of light into dark pixels. The double-pass system should also ideally have analyzers that operate over narrower bands and closely couple to their respective SLM's.